The present invention relates to an electro-optic element and manufacturing method therefor. In particular, the present invention relates to an electro-optic element which can be suitably used in optical waveguide modulators for optical communications or optical measurement, and to a manufacturing method therefore.
FIG. 7 is a cross-section showing an example of a conventional optical waveguide modulator.
This optical waveguide modulator comprises a ferroelectric substrate made of lithium niobate (LiNbO3) which is most generally and practically used in ferroelectric substrates for optical waveguide modulators.
In FIG. 7, reference 10 indicates a ferroelectric substrate comprising an X-cut lithium niobate. When this ferroelectric substrate 10 is cut along the X-axis direction (the crystallographic c-axis), which forms the principal optical axis, the Z-cut ferroelectric substrate 10 exhibits the pyroelectric effect (electric-optic effect). As shown in FIG. 7, the axis which exhibits the pyroelectric effect (electric-optic effect) of this ferroelectric substrate 10 is the Z-axis direction (the crystallographic c-axis) which forms the principal optical axis, and as shown in FIG. 7, it is a direction which is parallel to the face (in this specification this is referred to as the xe2x80x9cmain facexe2x80x9d) of the ferroelectric substrate 10 in which optical wave guides 2 and 2 are formed.
In the vicinity of the main face of the ferroelectric substrate 10, the optical waveguides 2 and 2 in which Ti has been thermally diffused are formed. Above that, a buffer layer 3 comprising SiO2 is formed. In addition, on to the buffer layer 3, electrodes 4 comprising Au are formed so that they lie parallel to the optical wave guides 2 and 2. A transition metal layer 5 comprising a transition metal such as Ti, Cr, or Ni is provided between these electrodes 4 and the buffer layer 3.
To manufacture this type of optical waveguide modulator, a method is used in which, first, optical waveguides 2 and 2 are formed in the main face of the ferroelectric substrate 10 by a thermal diffusion method. And the buffer layer 3 is formed by means of a vacuum deposition method, a sputtering method, or the like on the side of ferroelectric substrate 10 in which the optical waveguides 2 and 2 are formed. Next, a transition metal film and an Au film are successively formed across the entire surface of the buffer layer 3 by means of a vacuum deposition method. In addition, electrodes 4 are formed on this Au film by using an electrolytic plating method onto only the electrode formation regions which are regions on which the electrodes 4 and 4 are formed. Thereafter, the Au film and the transition metal film which remain between the electrodes 4 and 4 are removed by chemical etching, and the transition metal layer 5 is completed.
As described above, in this optical waveguide modulator, the direction of the Z-axis of the ferroelectric substrate 10 is parallel to the main face of the ferroelectric substrate 10.
In the optical waveguide modulator shown in FIG. 8, the direction of the Z-axis of the ferroelectric substrate 11 is orthogonal to the main face of the ferroelectric substrate 11. In this optical waveguide modulator, as the surrounding temperature changes, the generation of an electric charge due to the pyroelectric effect between the electrodes 4 and 4 is tend to occur. If the charge due to the pyroelectric effect accumulates between the electrodes 4 and 4, due to a random discharge phenomenon, or the like, the interaction between the electrodes 4 and 4 and the optical waveguides 2 and 2 becomes disturbed, and the modulation conditions of signals of the optical waveguide modulator become noticeably unstable.
Since the direction of the Z-axis of the ferroelectric substrate 10 of the optical waveguide modulator shown in FIG. 7 is parallel to the main face of the ferroelectric substrate 10, the charge which is generated by the pyroelectric effect does not substantially accumulate between the electrodes 4 and 4, and it has the advantage that problems due to the pyroelectric effect do not occur.
However, in this type of optical waveguide modulator, the buffer layer 3 is exposed between the electrodes 4 and 4. Therefore, there is the problem that the surface 3a at which the buffer layer 3 is exposed and the inner part of buffer layer 3 are easily contaminated by contaminants such as K, Ti, and Cr.
In particular, when the density of the buffer layer 3 is lowered by forming the buffer layer 3 using vacuum deposition method in order to regulate the properties of the optical waveguide modulator, it is easy for contaminants to penetrate into the buffer layer 3 through the exposed parts of the buffer layer 3, and this is a problem.
When the surface 3a of the buffer layer 3 and the inner part of the buffer layer 3 of the optical waveguide modulator become contaminated, dc drift may be generated. This dc drift is a phenomenon in which the electric current being applied to the electrodes 4 and 4 leaks through the buffer layer 3 due to the presence of mobile ions such as alkali ions, such as K and Na, and protons, the desired voltage is not applied effectively on the device, and this has a negative impact on the properties of the optical waveguide modulator.
Furthermore, when the contaminants of the buffer layer 3 reach the interface with the ferroelectric substrate 10 due to thermal treatments in the mounting process and the like, due to the contaminants, the chemical bonds of the buffer layer 3 comprising SiO2 are broken, the bonds which bond the ferroelectric substrate 10 comprising lithium niobate and the buffer layer 3 are reduced, and the problem results that the bonding strength between the two is remarkably weakened.
In addition, as another example of a conventional optical waveguide modulator, there is the optical waveguide modulator described in Japanese Unexamined Patent Application, Application No. Sho 60-214024 shown in FIG. 8.
In the same way as the optical waveguide modulator shown in FIG. 7, this optical waveguide modulator uses a ferroelectric substrate comprising lithium niobate (LiNbO3), however, the direction of the Z-axis which exhibits the pyroelectric effect of the ferroelectric substrate is different to that of the optical waveguide modulator shown in FIG. 7.
In FIG. 8, reference 11 indicates a ferroelectric substrate comprising lithium niobate having a Z-cut. The direction of the Z-axis which exhibits the pyroelectric effect of this ferroelectric substrate 11 is orthogonal to the main face of the ferroelectric substrate 11 in which the optical wave guides 2 and 2 are formed.
In the vicinity of the main face of the ferroelectric substrate 11, optical waveguides 2 and 2 comprising Ti are formed, and above that, a buffer layer 3 comprising SiO2 is formed. In addition, onto the buffer layer 3, a semi-conductive film 6 comprising an Si thin film or the like is provided. On this semi-conductive film 6, electrodes 4 comprising Au are formed so that they lie parallel to the optical waveguides 2 and 2.
In this type of optical waveguide modulator, the direction of the Z-axis of the ferroelectric substrate 11 is orthogonal to the main face of the ferroelectric substrate 11. Therefore, when the surrounding temperature changes, an electric charge is readily generated due to the pyroelectric effect between the electrodes 4 and 4. If the charge due to the pyroelectric effect accumulates between the electrodes 4 and 4, due to a random discharge phenomenon, or the like, the interaction between the electrodes 4 and 4 and the optical waveguides 2 and 2 becomes disordered, and the modulation conditions of signals of the optical waveguide modulator become noticeably unstable.
In the optical waveguide modulator shown in FIG. 8, a semi-conductive film 6 is provided on the buffer layer 3. This semi-conductive film 6 responds to the linear electric field induced by the pyroelectric effect, and it makes the charge due to the pyroelectric effect diffuse uniformly over the whole of the semi-conductive film 6. By doing this, even when a pyroelectric effect is generated, the random discharge phenomenon can be suppressed, and it is possible to stabilize the modulation conditions of the signals of the optical waveguide modulator.
However, in this type of optical waveguide modulator, the electrodes 4 and 4 are connected via the semi-conductive film 6. Therefore, when a radio frequency direct current bias is superimposed on and applied to the electrodes 4 and 4, there is the problem of lack of stability with respect to this application and, in particular, to the application of the direct current component. Consequently, this is not desirable when this optical waveguide modulator is operated for long periods of time.
As an optical waveguide modulator which solves these types of problems, the optical waveguide modulator described in Japanese Unexamined Patent Application, Application No. Sho 61-16307 and shown in FIG. 9 has been proposed.
The optical waveguide modulator shown in FIG. 9 differs from the optical waveguide modulator shown in FIG. 8 in that the semi-conductive film 61 is divided by portions on whose upper surfaces electrodes 4 and 4 are not formed.
In this type of optical waveguide modulator, because the semi-conductive film 61 is divided by portions on whose upper surfaces electrodes 4 and 4 are not formed, even when radio frequency direct current bias is superimposed and applied, there is no degradation in the stability with respect to the application conditions.
However, in this type of optical waveguide modulator, since the buffer layer 3 is exposed at those portions where the semi-conductive film 61 is divided between the electrodes 4 and 4, the surface 3b of the buffer layer 3 which is exposed and the inner portion of the buffer layer 3 are readily contaminated by contaminants, and the same problems as occur with the optical waveguide modulator shown in FIG. 7 occur.
In consideration of the above situations, the present invention aims to solve the above-mentioned problems and has an object of providing an electro-optic element in which the accumulation of charge generated due to the pyroelectric effect is less likely to occur, and in which contamination of the surface of the buffer layer and the inner portion of the buffer layer is less likely to occur.
In addition, the present invention has an object of providing a manufacturing method for the above-mentioned electro-optic element.
In order to achieve the above-mentioned objects, the electro-optic element of the present invention comprises a ferroelectric substrate comprising a single crystal having an electro-optic effect, in which an optical waveguide is formed by thermal diffusion of titanium in the main face, and in which an axis in which the above-mentioned electro-optic effect is induced is in a direction parallel to the above-mentioned main face; a heat treated buffer layer provided on the above-mentioned ferroelectric substrate on a side in which the above-mentioned optical waveguides are formed; electrodes provided on a part of the above-mentioned buffer layer; and a protective film for preventing the contamination of the buffer layer, which is provided on at least the region of the buffer layer on which the electrodes are not formed.
Here the xe2x80x9cmain facexe2x80x9d means the face of the ferroelectric substrate, in which the optical waveguides are formed.
The structure of the electro-optic element of the present invention is basically such that the axis in which the electro-optic effect of the ferroelectric substrate is induced is parallel to the main face of the ferroelectric substrate. This corresponds to the example of a conventional optical waveguide modulator shown in FIG. 7.
In the electro-optic element of the present invention, since the axis in which the electro-optic effect of the ferroelectric substrate is induced is parallel to the main face of the above-mentioned ferroelectric substrate, the charge which is generated by the electro-optic effect is essentially not stored between the electrodes, and problems resulting from the pyroelectric effect do not occur.
In addition, in the electro-optic element of the present invention, since a protective film is provided on at least the region of the above-mentioned buffer layer on which the above-mentioned electrodes are not formed, the surface of the buffer layer is not exposed. For this reason, the electro-optic element is such that it is difficult for the surface of the buffer layer and inner portion of the buffer layer to be contaminated.
Consequently, it is possible to prevent leakage of the electric current applied to the electrodes due to contaminants in the surface of the buffer layer and within the buffer layer, and it is possible to ensure the stability of the operation of the electro-optic element. Consequently, even if the direct current bias is superimposed and applied at radio frequency to the electrodes, it has excellent stability with respect to the application conditions. In addition, it is possible to prevent the generation of dc drift.
In addition, since it is difficult for the buffer layer to be contaminated, it is less likely for weakness in the bonding strength of the buffer layer and the ferroelectric substrate which results from contamination of the buffer layer to occur.
In the above-mentioned electro-optic element, it is preferable for the above-mentioned protective film to be provided over an entire surface of the above-mentioned buffer layer including regions on which the above-mentioned electrodes are formed.
By means of making this type of electro-optic element, the whole surface of the buffer layer is covered by the protective film and it is possible to prevent contamination of the buffer layer during the manufacturing process of the electro-optic element after the protective film has been formed, and it is possible to prevent even further penetration of contaminants into the buffer layer.
In addition, since the protective film is formed on the entire surface of the buffer layer, it is easier to form the protective film compared with a situation in which the protective film is formed on a part of the surface of the buffer layer.
In the above-mentioned electro-optic element, it is preferable for said protective film to be also provided on the side surfaces of said buffer layer with respect to the optical waveguide direction.
By making this type of electro-optic element, the side surfaces of the above-mentioned buffer layer with respect to the direction of the optical waveguides are covered by the protective film, and therefore, it is possible to prevent even more the penetration of contaminants into the buffer layer.
In addition, it is preferable for the above-mentioned protective film provided on the top of said buffer layer and the protective film provided on the side surfaces of said buffer layer with respect to the optical waveguide direction to be made of the same material.
By means of making this type of electro-optic element, it is possible to increase the chemical bonding strength of the protective film provided on the buffer layer and the protective film provided on the side surfaces of the buffer layer with respect to the optical waveguide direction. Furthermore, since the thermal expansion properties of the protective film provided on the top of the buffer layer and the protective film provided on the sides of the buffer layer are the same, the adhesiveness at the interface between the protective film provided on the top of the buffer layer and the protective film provided on the side surfaces of the buffer layer is thermally stable. As a result, it is possible to improve the effect of the protective film in preventing penetration of contaminants into the buffer layer, and it is possible to obtain even greater stability.
In the above-mentioned electro-optic element, it is preferable for the above-mentioned protective film to be an amorphous film.
Compared with a crystalline film, an amorphous film has structural continuity, and is dense. For this reason, by making the protective film an amorphous film, it is possible to form an electro-optic element in which it is even more difficult for the surface of the buffer layer and the inside of the buffer layer to be contaminated.
In addition, in the above-mentioned electro-optic element, it is preferable for the above-mentioned protective film to be electrically insulative.
Here, xe2x80x9celectrically insulativexe2x80x9d means a direct current resistance of greater than 20 Mxcexa9 and preferably greater than 50 Mxcexa9.
By making this type of electro-optic element, it is possible to prevent with even greater certainty leakage of the current applied to the electrodes and it is possible to ensure the stability of the operation of the electro-optic element. Consequently, it is possible to improve even further the effect of preventing the generation of dc drift.
In addition, it is preferable for the protective film to comprise at least one type selected from the group consisting of silicon (Si), silicon carbide (Sixe2x80x94C), and silicon nitride (Sixe2x80x94N).
In this type of electro-optic element, since the protective film comprises at least one type selected from the group consisting of silicon (Si), silicon carbide (Sixe2x80x94C), and silicon nitride (Sixe2x80x94N) which contain Si and which have high covalency, absorption of moisture is less likely to occur compared with SiO2. In addition, when the buffer layer comprises SiO2, the operation of the protective film in preventing the penetration of contaminants into the buffer layer is even better, and it is even more difficult for contamination of the buffer layer to occur.
In addition, since the protective film comprising at least one selected from the group consisting of silicon (Si), silicon carbide (Sixe2x80x94C), and silicon nitride (Sixe2x80x94N) does not contain oxygen, even when a transition metal layer is formed from a chemically active transition metal such as Ti on the protective film, there is no oxidative degradation of the transition metal layer and it is possible to obtain a stable film bonding strength.
Furthermore, it is preferable for the above-mentioned ferroelectric substrate of the electro-optic element to comprise lithium niobate.
With single crystals of lithium niobate, it is possible to realize large size integrated devices by making the ferroelectric substrate from lithium niobate since it is comparatively easy to grow large size crystals.
In addition, since the Curie point of single crystals of lithium niobate is approximately as high as 1000xc2x0 C., the degree of freedom for temperatures in the manufacturing process of electro-optic element is larger.
In addition, in order to overcome the above-mentioned problems, the manufacturing method for electro-optic elements of the present invention comprises a step of forming optical waveguides in a surface of a ferroelectric substrate comprising a single crystal having an electro-optic effect, in which the axis in which the above-mentioned electro-optic effect is induced is parallel to the main face; a step of forming a buffer layer on top of the above-mentioned ferroelectric substrate on the side in which the above-mentioned optical waveguides are formed; a step of forming a protective film for preventing contamination of the above-mentioned buffer layer on at least the regions of the above-mentioned buffer layer which are not the electrode forming regions; and a step of forming electrodes on the above-mentioned electrode forming regions.
By means of this type of manufacturing method for the electro-optic element, it is possible to easily obtain the above-mentioned electro-optic element.